The invention relates to a method and apparatus primarily adapted for treating a comminuted cellulosic fibrous material suspension, such as pulp for the production of paper. The invention primarily relates to the thickening of the suspension from a relatively low consistency (e.g., about 11/2%), to a higher, more desirable consistency (e.g., between about 9-14%).
The invention is particularly adapted for use with the general apparatus illustrated and described in U.S. Pat. No. 4,441,224, the disclosure of which is hereby incorporated by reference herein. In that application, a simple and effective method and apparatus are provided for the treatment of a suspension, such as pulp suitable for use in making paper, utilizing stationary screens. The utilization of stationary screens as described in said application greatly simplifies the suspension treating procedure, yet no significant screen clogging--which would result in non-uniform treatment--occurs.
The basic apparatus disclosed in said copending application Ser. No. 374,763 includes an upright cylindrical vessel having a vertical axis, a plurality of annular, stationary screens disposed within the vessel and spaced from each other and from the axis; a plurality of radially extending extraction arms physically and hydraulically connected to the stationary annular screens, the extraction arms partitioning the vessel into a plurality of radial segments of substantially the same cross-sectional area, and the screens partitioning each radial segment into a plurality of vertically extending channels; and a suspension inlet means mounted below the extraction arms and the screens. Such a structure also includes a central shaft mounted for rotation within the vessel about its axis, and provides that each screen segment of each of the screen structures in a particular radial segment is hydraulically connected to the same extraction arm, and only a single extraction arm. A plurality of extraction conduits, one extending from each of the extraction arms, are provided, as well as extraction valve means for controlling the flow of liquid from each of the extraction arms.
According to the present invention, a method and apparatus are provided for thickening of the suspension, preferably between its introduction into the inlet means of the vessel described above, and prior to moving into operative relationship with the stationary annular screens of the vessel described above. The thickening means comprises a plurality of rotatable hollow arms, rotatable about said axis, the arms mounting a plurality of annular screens thereon, which screens are hydraulically connected to the hollow arms. As with the extraction arms and stationary screens, each screen segment of each of the screen structures in a particular radial segment defined by the hollow radially extending arm is hydraulically connected to the same hollow arm, and only a single hollow arm. A plurality of filtrate conduits, one extending from each of the hollow arms, are provided, as well as filtrate valve means for controlling the flow of filtrate from each of the hollow arms. Preferably, the number of rotatable hollow arms is half the number of extraction arms, and the hollow arms are rotated by a second, hollow, shaft in a direction--preferably but not necessarily opposite to the direction of rotation of the central shaft, and at a higher speed.
In the practice of the general method according to the present invention, the thickening of suspension from a low consistency (e.g., about 11/2%) to a higher consistency (e.g., between about 9-14%) is practiced by rotating the second shaft connected to the hollow rotatable arms--with annular screens connected thereto--in the opposite direction as the central shaft. The central shaft is attached to a mechanism for extracting a portion of the entire radial extent of the suspension at the cross-sectional area of the top of the suspension column in the vessel, and at that part of the top of the column preceding the radial segment to which the suspension is being introduced after being thickened at the bottom of the column.
The present invention also relates to the thickening apparatus, per se, for accomplishing suspension thickening.
It is the primary object of the present invention to provide a simple and effective mechanism for the thickening of a suspension, such as pulp, primarily in a system utilizing stationary screens for the subsequent treatment of the suspension. This and other objects of the present invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.